1. Field of the Invention
This invention is generally related to the reactivation of catalysts, more particularly to the reactivation of catalysts through engine management strategies.
2. Related Art
Automobile manufacturers employ a number of engine management strategies in order to improve fuel economy. For engines controlled to operate at the stoichiometric air-to-fuel ratio (.lambda.), the fuel injectors are shut down during deceleration driving modes. Such a strategy is called a "fuel-cut" or "lean-out" strategy. U.S. Pat. No. 4,214,307 describes a deceleration lean-out feature for electronic fuel management systems the disclosure of which is incorporated by reference. This feature provides for increasing the air/fuel ratio upon a deceleration. By creating this lean-out feature, breakthrough of a rich air/fuel ratio is purported to be avoided thereby lessening unbalancing and/or reduced efficiency of the catalytic converter.
Greater fuel economy benefits compared to the fuel-cut or lean-out modes are derived by operating the engine under lean, i.e., excess oxygen, air-to-fuel conditions. Here, lean operation can be employed only under certain driving modes, e.g., cruise modes; or under almost all driving modes, e.g., with a lean-burn engine.
A problem associated with automobile catalytic converters used on stoichiometrically controlled vehicles is their known susceptibility to deactivate when exposed to high temperature, excess oxygen conditions. For example, platinum crystallites are known to sinter under these conditions, thereby reducing the area available for catalysis. Rhodium crystallites oxidize to form a much lower activity rhodium oxide. In addition, rhodium reacts with materials that are used to disperse the metal such as alumina at temperatures in excess of 800.degree. C. The resultant rhodium aluminate product is essentially inactive for catalysis of NOx.
These catalyst deactivation modes become particularly severe for automobile engines that are designed to run lean either part or all of the time such as in partial or full lean-burn modes. In fact the problem of catalyst deactivation is more pronounced as the inherent excess oxygen conditions of the lean-burn modes are more prevalent as compared with stoichiometric operation of an engine. Thus, in lean-burn environments there is more of a need for proper reactivation of the catalyst.
EP 503 882 describes an exhaust gas purification system for lean-burn engines which includes hydrocarbon injection means which is activated when NOx catalyst temperatures reach a predetermined minimum. The injected hydrocarbon is purported to be partially oxidized to form radicals at the lower NOx catalyst temperature and held within the cells of the NOx catalyst. When the NOx catalyst temperature rises, the stored hydrocarbon is released to promote high NOx purification at higher NOx catalyst temperatures. However, EP 503 882 contains no disclosure with regard to regeneration of the rhodium component of the rhodium-containing catalyst as disclosed and claimed by the present invention.
EP 580,389 describes an exhaust gas purification apparatus capable of recovering an NOx absorbent poisoned by sulfur oxides (SOx). In contrast to the present invention, EP 580,389 teaches against the use of fuel cut means, because at high temperature conditions (i.e., exhaust gas temperatures greater than 550.degree. C.) SOx poisioning of the NOx absorbent is promoted.
The present invention offers an advance over known engine strategies in being able to reactivate the rhodium function of engine exhaust catalysts.